Touch sensors are transparent or opaque input devices for computers and other electronic systems. As the name suggests, touch sensors are activated by touch, either from a user's finger, or a stylus or some other device. A transparent touch sensor, and specifically a touchscreen, is used in conjunction with a display device, such as cathode ray tube (CRT), liquid crystal display (LCD), plasma, electroluminescent, or other type of display, to form a touch display. These touch displays are increasingly used in commercial applications, such as restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, pagers, cellular phones, personal digital assistants, and video games.
The dominant touch technologies presently in use are resistive, capacitive, infrared, and acoustic technologies. Touchscreens incorporating these technologies have delivered high standards of performance at competitive prices. All are transparent devices that respond to a touch by transmitting the touch position coordinates to a host computer. Acoustic touchscreens, also known as ultrasonic touchscreens, have competed effectively with these other touch technologies. This is due in large part to the ability of acoustic touchscreens to handle demanding applications with high transparency and high resolution touch performance, while providing a durable touch surface.
Typically, an acoustic touchscreen comprises a touch sensitive substrate in which an acoustic wave is propagated. When a touch occurs on the substrate surface, it results in the absorption of at least a portion of the wave energy being propagated across the substrate. The touch position is determined using electronic circuitry to locate the absorption position in an XY coordinate system that is conceptually and invisibly superimposed onto the touchscreen. In essence, this is accomplished by recording the time the wave is initially propagated and the time at which a touch induced attenuation in the amplitude of the wave occurs. The difference in these times can then be used, together with the known speed of the wave through the substrate, to determine the precise location of the touch.
A transparent touch sensor, and specifically a touchscreen, is generally placed over a display device, such as cathode ray tube (CRT), liquid crystal display (LCD), plasma, electroluminescent, or other type of display. Alternatively, the touchscreen can be constructed directly on the front surface of the display device, so that the surface of the display device is touch sensitive. This latter construction is desirable because it eliminates a piece of glass or other material between the viewer and the display device, increasing the perceived display brightness and contrast ratio. Also, there are economic advantages in dispensing with an overlay glass and not having to modify the chassis of the display device to make room for the overlay glass.
The acoustic touchscreen comprises an acoustic substrate and transducers, which are elements that convert energy from one form to another. For example, a transmit transducer may receive a tone burst from associated electronic circuitry and then emit an acoustic wave across the substrate. A receive transducer may receive a transmitted acoustic wave from the substrate and generate an electronic signal that is transmitted to associated electronic circuitry for processing.
Various types of acoustic transducer assemblies are known. The most common types used in acoustic touchscreens are wedge transducer assemblies, grating transducer assemblies, and edge transducers.
FIG. 1(a) illustrates a typical wedge transducer assembly 10a, which utilizes the phenomenon that acoustic waves are refracted when they are obliquely incident on a boundary surface of different media with appropriately differing wave velocities. Based on this principle, the wedge transducer assembly 10a consists of a plastic wedge 12 with its hypotenuse adhered to the front surface 18 of the acoustic substrate 16, which is composed of a different material than that of the wedge 12, e.g., glass. The wedge transducer assembly 10a also comprises a transducer, and specifically a piezoelectric element 14, mounted to a side of the wedge 12 other than the hypotenuse. As illustrated by the arrows, the piezoelectric element 14 couples to a bulk wave in the wedge 12, which propagates at the critical angle, i.e., the “wedge angle,” to refract to or from a horizontally propagating wave in the substrate 16.
FIG. 1(b) illustrates a typical grating transducer assembly 10b, which comprises a grating 22 composed of perturbation elements 24, which are aligned in parallel strips along front substrate surface 18. The grating transducer assembly 10b also comprises a transducer, and specifically a piezoelectric element 26, mounted on a rear surface 28 of the substrate 16 opposite the front substrate surface 18. As illustrated by the arrows, the piezoelectric element 26 couples to a bulk wave in the substrate 16. This bulk wave couples, via the grating 22, to two oppositely traveling horizontally propagating waves in the substrate 18. Further details regarding the structure and use of grating transducers are disclosed in U.S. Pat. No. 6,091,406, which is expressly incorporated herein by reference.
FIG. 1(c) illustrates a typical edge transducer 10c, which comprises a piezoelectric element 32 mounted directly on an edge 34 of the substrate 16 in such a manner that an acoustic wave with appreciable power at the front substrate surface 18 is generated. The interface thus serves the mechanical function of connecting the piezoelectric element 32 to the substrate 16, as well as the acoustic function of coupling to a horizontally propagating wave in the substrate 16, as illustrated by the arrows. Further details regarding the structure and use of edge transducers to excite horizontally polarized shear waves are disclosed in U.S. Pat. No. 5,177,327, which is expressly incorporated herein by reference.
Ultimately, the selection of which transducer type to use will depend, at least in part, on the structural environment in which the touchscreen is to be mounted. For example, selection of the transducer type may depend on whether the acoustic substrate is either overlaid on the front panel of a display device to form a separate faceplate, or incorporated directly into the front panel of the display device. Selection of the transducer type may also depend on the shape of the acoustic substrate, e.g., whether it is curved or flat.
For example, FIG. 2 illustrates a touch display 50 that comprises a display device 52 and an acoustic substrate 54 that is overlaid onto the display device 52. The display device 52 has a curved front panel 56, such as in a typical cathode ray tube, and the acoustic substrate 54 has a corresponding curved shape. Due to the curved geometry of the acoustic substrate 54, a space exists between the substrate 54 and a bezel 58 covering the periphery of the substrate 54. In this case, a wedge transducer assembly 10a, even with its relatively high profile, can be conveniently mounted on the front surface 60 of the substrate 54 within this space. Thus, wedge transducer assemblies 10a may be used where it is possible or desirable to mount a transducer on the front surface 60 of the acoustic substrate 54.
FIG. 3 illustrates a touch display 70 that also comprises a display device 72 and an acoustic substrate 74 that is overlaid onto the display device 72. The display device 72, however, has a flat front panel 76, such as a liquid crystal display, a flat CRT or a plasma display, and the acoustic substrate 74 is also flat. As a result, there is no or very little clearance between the substrate 74 and the bezel 58. In this case, a grating transducer assembly 10b can be used despite the minimal clearance provided. The gratings 22 of the transducer assembly 10b, which have a relatively low profile, can be located on the front surface 80 of the substrate 74 within the minimal space provided between the bezel 58 and the substrate 74. The piezoelectric element 26 can be located on the rear surface 82 of the substrate 74. The rear substrate surface 82 may be beveled or inclined in order to provide clearance between the piezoelectric element 26 and the front panel 76 of the display device 72.
In touch displays where there is peripheral space available between the bezel 58 and the edges of the acoustic substrate, an edge transducer 10c can be mounted to the substrate in this space. However, the requirement of a carefully machined vertical surface may add significant cost to this approach. Furthermore, if coupling to Rayleigh waves is desired, edge transducers become more complex and thus less desirable.
Although a touchscreen manufacturer can typically find a viable solution when selectively incorporating the above-described transducers 10 within an acoustic substrate that forms a separate faceplate, such may not be the case when the acoustic substrate forms the front panel of the display device, i.e., the display device, itself, has a touch sensitive front panel. For example, the piezoelectric element of a grating transducer assembly must be placed on the rear surface of the acoustic substrate—an option not available when the substrate forms the front panel of the display device. In the case where the display device has touch sensitive front panel that is flat, e.g., a CRT or 50″ plasma-display, mounting of a wedge transducer assembly on the front surface of the display may be difficult, often resulting in mechanical interference between the bezel and the transducer. This interference may impede the proper functioning of the transducer, or worse yet, damage either the transducer or the bezel. Much more so than the case where the acoustic substrate forms a separate faceplate, it may be very difficult to provide a vertical machined surface for an edge transducer.
Often, a touchscreen manufacturer does not have the option to modify the housing in which the display device is enclosed. In building a touchscreen that forms the faceplate of a display device, the touchscreen manufacturer normally does not manufacture the display device itself. Rather, the manufacturer works with the display device, as supplied by a monitor manufacturer. Since it is often impractical for the touchscreen manufacturer to replace the supplied housing with a new housing, the manufacturer must adapt to whatever space is available between the supplied housing and the display device for accommodating the touchscreen elements. Even where the touchscreen manufacturer has design control over the bezel, mechanical interference with the transducers often forces a reduction in the dimensions of the bezel opening that prevents one from utilizing the full available display area of the display device.
There thus remains a need to provide a relatively low-profile transducer that can be mounted on the front surface of an acoustic substrate.